2 Answers
2

EDIT: I took a late look at some pictures of the wheel you mentioned (Shimano WH-R550). It looks quite different from the regular mtb shimano hubs, so please consider the following answer as applying to regular mountain-bike shimano hubs.

From my experience with shimano hubs, there are two parts:

The locknut, which face outwards and make contact with the fork/frame, is usually 17mm. Since you need to tighten it with the other side fixed, you can use a 17mm wrench at one side and a 18mm wrench at the other if you don't want to have two (big/expensive) 17mm wrenches.

Cones are just inside the locknuts and might be 13 or 14, or even different. Usually you need a dedicated cone wrench since normal wrenches are too thick to fit in the narrow fitting in the cone.

Some shimano models have a double-size cone, to allow use of two wrenches of different sizes, so it helps folks who have only one size of cone wrench (Shimano is rarely so considerate).

I waited (and suffered) too much before buying my cone wrenches. If I knew how much a difference they make, I'd have bought them at the same time I bought the bike!

locknut is equal to 18mm, cone nut can be 17mm, i bought one for 17mm and 18mm cone wrench, since 16mm doesn't fit and I can not use variable wrench i have, too thick, i wish i have a thin plier (2mm alligator) it will work....for holding when i loosen or tighten the lock nut..
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Rick AntFeb 16 '12 at 2:34